Population: 126,382
How could the air not be clean in this South Dakota city? It's located near vast wilderness areas, within 25 miles of Badlands National Park and on the edge of the Black Hills National Forest.
"The winds come out of the Black Hills, scout the basin and blow everything eastward," said Brett Limbaugh, director of community planning and development for the city.
Only 120,000 people live in the entire Rapid City metro area and the nearest town of any real size, Sheridan, Wyo., is about 250 miles away.
Rapid City is also trying to keep a tight lid on the builders looking to develop in the area. They're required to obtain air quality permits and use dust containment systems.
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